Marcus Ranum pointed me towards a talk by Bill Cheswick (.ppt) on IPv6 transition realities. It mentions several problems that might prevent IPv6 adoption, like unreasonable demands on routers, hosts which can pick a new IPv6 address for every connection, and other issues.

I think it only takes a little imagination to think of scads of uses of the Internet that are impossible today because of the limitations of IPv4 address space and the poor topology of our IPv4 internet, mostly thanks to NAT. Honestly, IPv6 is going to be widely deployed whether North America wants it or not, so we'd better play along instead of being left behind.

I don't really see how the US has much choice in the matter. I agree with the above poster that the US needs to get moving or get left behind. China and others are on their way. Besides, all modern OSs support IPv6 and a lot of recent network devices do too.